Grant: Discovery Grants Program - Individual

A whole muscle may contain several hundreds of thousands of individual muscle fibers. These fibers are grouped into functional units (10 - 1000 fibers) activated by a single motor neuron. A single motor neuron and all the muscle fibers that it activates is a call a motor unit (MU). The MU is the basic building block of force production. Force can be increased by recruiting additional MUs, or by increasing the number of times per second (frequency) that each MU is activated. Activating MUs more frequently allows the forces generated by the MUs to overlap in time to a greater degree and sum up more efficiently. Similarly, the nervous system can also recruit groups of MUs more synchronously so that there is a near perfect summation of forces. When a MU is recruited by the nervous system, it generates electrical activity that can be monitored by implanting an electrode into the muscle. Unfortunately, indwelling electrodes record from a limited volume of tissue, which is not representative of the whole muscle. However, the electrical activity from many MUs can sum up sufficiently to be detected from the skin surface, using surface electromyography (sEMG). This similar to the use of electrocardiography (ECG) to record the electrical activity of heart muscle from the skin surface. Surface EMG has the advantage of recording the electrical activity of a large population of MUs, but the electrical activity of individual MUs ""obscured"". The perplexing dilemma is that sEMG electrodes are the easiest way to measure muscle electrical activity during normal human movement. Yet, the ability of the sEMG signal to provide insight into the neural mechanisms underlying the gradation of muscle force is limited. The proposed series of studies combines indwelling and surface methodologies in a unified approach towards understanding how the nervous system regulates muscle force.